1. Field
This invention relates to methods, apparatus and systems for distributing digital data encoded in a way to provide temporal scalability.
2. Background
Due to the explosive growth and great success of the Internet and wireless communication, as well as increasing demand for multimedia services, streaming media over the Internet and mobile/wireless channels has drawn tremendous attention. In heterogeneous Internet Protocol (IP) networks, video is provided by a server and can be streamed by one or more clients. Wired connections include dial-up, integrated services digital network (ISDN), cable, digital subscriber line protocols (collectively referred to as xDSL), fiber, local area networks (LAN), wide area networks (WAN) and others. The transmission mode can be either uni-cast or multi-cast. The variety of individual client devices, including personal digital assistant (PDA), laptop, desktop, set-top box, TV, HDTV, mobile phone and others, requires bitstreams of different bandwidths simultaneously for the same content. The connection bandwidth could vary quickly with the time (from 9.6 kbps to 100 Mbps and above), and can be faster than a server's reaction.
Similar to the heterogeneous IP network is mobile/wireless communication. Transport of multimedia content over mobile/wireless channels is very challenging because these channels are often severely impaired due to multi-path fading, shadowing, inter-symbol interference, and noise disturbances. Some other reasons such as mobility and competing traffic also cause bandwidth variations and loss. The channel noise and the number of users being served determine the time-varying property of channel environments. In addition to environmental conditions, the destination network can vary from second to third generation cellular networks to broadband data-only networks due to geographic location as well as mobile roaming. All these variables that affect the available bandwidth call for adaptive rate adjustment of the transmission of multimedia content, even on the fly. Thus, successful transmission of video over heterogeneous wired/wireless networks requires efficient coding, as well as adaptability to varying network conditions, device characteristics, and user preferences, while also being resilient to losses.
To meet different user requirements and to adapt to channel variation, one could generate multiple independent versions of bitstreams, each meeting one class of constraints based on transmission bandwidth, user display and computational capability, but this is not efficient for server storage and multicast application. In scalable coding, where a single macro-bitstream accommodating high-end users is built at the server, the bitstreams for low-end applications are just embedded as subsets of the macro-bitstream. As such, a single bitstream can be adapted to diverse application environments by selectively transmitting sub-bitstreams. Another advantage provided by scalable coding is for robust video transmissions on error prone channels. Error protection and error concealment can be easily handled. A more reliable transmission channel or a better error protection can be applied to base-layer bits that contain the most significant information.
There are spatial, temporal and signal to noise ratio (SNR) scalabilities in hybrid coders like MPEG-1, MPEG-2, MPEG-4 (collectively referred to as MPEG-x), H.261, H.262, H.263, and H.264 (collectively referred to as H.26x). In hybrid coding, temporal redundancy is removed by motion-compensated prediction (MCP). A video is typically divided into a series of groups of pictures (GOP), where each GOP begins with an intra-coded frame (I) followed by an arrangement of forward predicted frames (P) and bidirectional predicted frames (B). Both P-frames and B-frames are inter-frames. The B frame is the key to temporal scalability in most MPEG like coders. However, some profiles, such as the MPEG-4 Simple profile and the H.264 Baseline Profile do not support B frames.
In MPEG-4, profiles and levels provide a means of defining subsets of the syntax and semantics based on the decoder capabilities required to decode a particular bitstream. A profile is a defined sub-set of the entire bitstream syntax. A level is a defined set of constraints imposed on parameters in the bitstream. For any given profile, levels generally correspond to decoder processing load and memory capability. So profiles and levels specify restrictions on bitstreams and hence place limits on the capabilities of decoding the bitstreams. In general, a decoder shall be deemed to be conformant to a given profile at a given level if it is able to properly decode all allowed values of all syntactic elements as specified by that profile at that level.
It is an object of the present invention to provide a method and apparatus providing simple yet effective temporal scalability that also conforms to the MPEG-4 Simple Profile and the H.264 Baseline Profile. The MPEG-4 standard is described in ISO/IEC 14496-2. The H.264 standard is described in [ISO/IEC 14496-10]